Need water, builder's sand, aggregate, cement, wheelbarrows, form, power, shovels, trowels.... Very noisy; very hard work. A 'development site' is not going to be using bagged mixes of 'Quikcrete'; yards-sized batches are ordered from ready mix concrete companies. Unformed/ unfinished blobs of concrete of any significant size laying on the ground would be very unusual. Finishing concrete requires light. Clean-up--the tools must be cleaned up. Pronto. Way too complicated, too time consuming, too noisy, heavy & difficult work. And you end up with cement on your clothes & footwear.
You'd have to have someone who knew a little about what they were doing re: the concrete, etc. wouldn't you? I don't see any of the suspects (other than maybe PVDS) of having any experience like this.